Who Could Replace John Bolton?

President Trump on Tuesday announced the departure of John R. Bolton as his national security adviser, the third person to hold the job since the beginning of the Trump administration. Though the White House has said Mr. Bolton’s current deputy, Charles M. Kupperman, will take over in the interim, Mr. Trump has said he will announce a successor next week.

A guessing game immediately began among the president’s formal and informal advisers about who still left in the president’s orbit might get the job.

The expanding list of possibilities, generated by those hoping to promote their allies or harm their enemies, included Fred Fleitz, Mr. Bolton’s former chief of staff; Keith Kellogg, a retired lieutenant general and a former acting national security adviser; Jack Keane, a retired Army vice chairman currently advising the vice president on national security; Robert Blair, an adviser to Mick Mulvaney, the acting chief of staff; and Robert C. O’Brien, the administration’s hostage envoy who called Mr. Trump the greatest hostage negotiator in American history.

As the administration begins to resemble a game of reverse musical chairs — too many open slots without enough loyalists to fill them — a short list of plausible replacements emerged.

The acting Adviser

Charles M. Kupperman

Mr. Kupperman, a former Reagan administration official and defense contracting executive, is a longtime Bolton associate. Known by many national security officials by his nickname, “Kupperware,” for his blandness, Mr. Kupperman, 68, was appointed in January as deputy national security adviser under Mr. Bolton.

Shortly after Mr. Bolton left the White House on Tuesday, Hogan Gidley, a deputy White House spokesman, told reporters that Mr. Kupperman would serve as Mr. Bolton’s acting successor. Acting officials have a way of sticking around in this administration for indefinite lengths of time, but Mr. Kupperman’s track record as someone ensconced in Mr. Bolton’s inner circle could shorten his tenure.

Still, the president appreciated Mr. Kupperman’s just-the-facts style compared with Mr. Bolton’s often ideologically charged delivery: If Mr. Trump had to have a national security brief concerning long-term planning, he preferred it from Mr. Kupperman as opposed to Mr. Bolton, according to a person with knowledge of that process.

The representative to North Korea

Stephen E. Biegun

Mr. Biegun, the United States’ special representative for North Korea, had a firsthand window into the clashes between Mr. Bolton, who never wavered from a hawkish, hard-line stance on North Korea, and the president, who has tried to use a charm offensive to persuade Kim Jong-un, the North Korean leader, down a path to denuclearization.

Mr. Biegun is considered a capable technocrat rather than a big-ideas person, unlike Mr. Bolton, who had firm ideological views that shaped his policy positions. Recently Mr. Biegun has been in closer alignment with Secretary of State Mike Pompeo and Mr. Trump than with the hard-line, anti-North Korea views of Mr. Bolton.

“The challenge is to find a way through diplomacy to resolve it,” Mr. Biegun said. “The president has made it clear that short-range missiles don’t make him happy, but it’s not going to disrupt our efforts in order to engage diplomatically to resolve the very issues that we are referring to.”

This summer, Mr. Biegun was initially floated internally as a possibility to succeed Jon Huntsman Jr., who resigned in August as the administration’s ambassador to Russia. That job ultimately went to John Sullivan, the deputy secretary of state under Mr. Pompeo.

The White House did not respond to a request for comment about whether Mr. Biegun had recently interviewed with the president for the job of national security adviser.

Mr. Biegun also served as an executive secretary of the National Security Council under President George W. Bush. In August 2001, Mr. Biegun was with the president, then on vacation at his ranch in Texas, when Mr. Bush received a daily brief containing an article with the title “Bin Laden Determined to Strike in U.S.”

The administration’s Iran representative

Brian H. Hook

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Brian H. Hook, the special representative to Iran, is one of the remaining appointees of the Rex W. Tillerson era.CreditGabriella Demczuk for The New York Times

Mr. Hook, 51, is also said to be in contention to succeed Mr. Bolton. He is the administration’s special representative for Iran and a senior adviser to Mr.Pompeo.

Mr. Hook, a lawyer brought into the State Department under Rex W. Tillerson, is one of the remaining survivors from that era. An administration official familiar with Mr. Hook’s relationship with Mr. Trump said that the two “interact on Iran” and that “the president is happy with how the strategy is going there.”

He would also probably have the support of Jared Kushner, the president’s son-in-law and senior adviser, who has tried to push his allies into high-profile administration positions before. But Mr. Hook could already be engaged. He has stepped up to take on Mr. Kushner’s Middle East portfolio as Jason Greenblatt, the co-architect of the administration’s peace plan for that region, prepares to leave.

Another Fox News fixture

Douglas Macgregor

Mr. Trump is almost certainly familiar with Mr. Macgregor, a retired Army colonel who has written several books on reorganizing the military. But more important to Mr. Trump, he also appears frequently on one of the president’s favorite Fox programs, “Tucker Carlson Tonight.”

In June, when Mr. Trump decided at the last minute to call off a round of strikes against Iran, he had listened to Mr. Carlson’s assertion that a strike could prove politically fatal. A frequent guest on the show that week was Mr. Macgregor, who backed up that rationale.

Reached by telephone on Tuesday afternoon, Mr. Macgregor seemed to expect the call. “It’s no comment, no comment, no comment,” he said, declining to say whether he had talked to the White House about Mr. Bolton’s job.

Either way, solid television performances may not be the safest route to Mr. Trump’s good graces. The president had also liked the look of Mr. Bolton’s fiery Fox News performances before he hired him for the national security adviser post.

The wild card

Richard Grenell

Mr. Grenell, the American ambassador to Germany, is personally liked by the president. At times, he has emulated Mr. Trump’s brash diplomatic style. Shortly after beginning his post in Germany, he elicited the annoyance of politicians there by admonishing any German companies doing business with Iran.

Mr. Grenell, 52, who is gay, is perhaps best known for enthusiastically defending the president’s position on gay rights, even as the Trump administration has taken steps to roll back civil rights for gay and transgender people. He has also led an effort to decriminalize homosexuality around the globe.

Throughout his tenure, Mr. Grenell has told his allies that he has been considered for several high-ranking positions — this year, his name was floated as a prospective nominee for ambassador to the United Nations, a position that Kelly Knight Kraft, the ambassador to Canada at the time, ultimately filled. He expects to be interviewed for Mr. Bolton’s job, according to a person with knowledge of the planning process.

The really wild card

Lt. Gen. H.R. McMaster

General McMaster, who was ousted last year weeks after a furious tweetstorm from Mr. Trump over his comment that there was “incontrovertible” evidence of Russian election interference, has received at least one phone call from the president on matters of national security, according to a report from NBC News and confirmed by The New York Times.

The chances he is offered the job? “Less than zero,” according to a person familiar with his historically fraught relationship with Mr. Trump.

In any other administration, that would mean he wouldn’t have a chance.

Another possibility from the McMaster era could be Ricky Waddell, a former deputy national security adviser who left the White House last year. In an interview on Tuesday with Fox News, Senator Lindsey Graham, Republican of South Carolina, said that Mr. Trump had mentioned Mr. Waddell by name, along with Mr. Hook and Mr. Kellogg.